1. Field of the Invention
The present invention pertains to microwave switching devices, and more particularly to controlled switching devices utilizing novel field-effect devices to provide such switching.
2. Description of the Prior Art
Prior art devices typically use PIN diodes for switching at microwave frequencies. Such use is documented in Microwave Diode Control Devices, R. V. Garver, published by Artech House, Inc., Dedham, MA., 1976 and incorporated herein by reference.
The use of such PIN diodes, however, is typically associated with several disadvantages, including a long switching transition time delay caused by the thickness of the intrinsic layer, typically in the range of 1-10 microns. Additionally, biasing of such diodes requires the application of a bias current to the device being switched in order to maintain the diode in its conductive state, and the application of a reverse biasing voltage to keep the diode nonconducting, as required.
Because of the widespread application of such microwave diode control devices to new systems and their inclusion in integrated circuits, improved performance of the devices is widely sought. A comprehensive bibliography of development and application of microwave field-effect-transistors (FET's) is provided in Liechti, "Microwave Field-Effect Transistors", I.E.E.E. Trans. Microwave Theory Tech., vol. MTT-24, pp. 279-300, 1976. The reference includes a summary of microwave FET operation, but does not include a device as disclosed herein.
Dobratz et al disclose the connection of a conventional MOSFET to a circulator at 4-8 GHz to provide 180.degree. phase shift in "Gigabit-Rate GaAs FET RF Phase Modulators", 1978 I.E.E.E. Int. Solid-State Circuits Conf., Dig. Tech. Papers, pp. 76-77. While the phase shifter demonstrates high speed and efficient modulation, the insertion loss is higher than desired, exceeding 1 dB. Dual gate MOSFET's have demonstrated 60 dB dynamic switching range and high speed at 4-8 GHz, as disclosed by Goel et al, "A 4-8 GHz Dual Gate Amplifier", 1978 I.E.E.E. Int. Solid-State Circuits Conf., Dig. Tech. Papers, pp. 126-127.
Where FET devices are used as amplifiers, particularly at high input power levels, current limiting is observed when the source-to-drain voltage approximately equals the gate voltage. This mechanism leads to limitation of the power output of microwave amplifiers.
Fukuda et al optimize an amplifier to provide such limiter operation at 10 GHz. Such limiting is desirable in several phase modulated systems in order to reduce amplitude modulation on the carrier. 1977 I.E.E.E. Int. Microwave Symp. Dig. Tech. Papers, pp. 240-242.